For a long time, please give pointers to heroes!

iceman5230

There is a rand.jsp page that generates a verification code:

// Take a randomly generated authentication code (4 digits)
String sRand = "";
for (int i = 0; i <4; i ++) {
    String rand = String.valueOf (random.nextInt (10));
    sRand + = rand;
    // Display the authentication code in the image
    g.drawString (rand, 11 * i + 1,12);
}

// session saves rand
session.setAttribute ("rand", sRand);



Landing page index.jsp:


<form action = "<% = request.getContextPath ()%> / login.do" name = "loginform" method = "post">

      <td> User: <input type = textbox name = "username" value = "" /> </ td>
      <td> Password: <input type = password name = "password" value = "" /> </ td>
      <td> Verification code: <input type = textbox name = "randt" value = "" />
                  <img border = 0 src = "rand.jsp"> </ td>

     <td>
     <input type = submit value = "login" name = "loginsub">
     <input type = reset value = "override" name = "loginres">
     </ td>

     <% = session.getAttribute ("rand")%>



    Why does the value of session.getAttribute ("rand") differ from the value of <img border = 0 src = "rand.jsp>? Session.getAttribute (" rand ") shows <img border = 0 The previous value of src = "rand.jsp>, if it is the first session.getAttribute (" rand ") shows null, but <img border = 0 src =" rand.jsp> has a value
    But in LoginAction
         session.getAttribute ("rand");
    String rand = (String) session.getAttribute ("rand");
    String randt = forms.getRandt ();
    if (randt.equals (rand))
     {equal}
         else {不等}
They are equal, but they will be prompted when they are filled in for the first time, and they will be equal in the future.

Ask you heroes, what is the reason for this? What should I do?
First, thank you!

wh_family

Disable page caching, it displays the value of the previous cache every time!

iceman5230

Isn't it

// Set the page not to cache
response.setHeader ("Pragma", "No-cache");
response.setHeader ("Cache-Control", "no-cache");
response.setDateHeader ("Expires", 0);

Did you write the code above?

qiqituo

Cache less once?

钢铁男子

Try to write like this

for (int i = 0; i <4; i ++) {
    String rand = String.valueOf (random.nextInt (10));
    String sRand + = rand;
    // Display the authentication code in the image
    g.drawString (rand, 11 * i + 1,12);
}

// session save rand
session.setAttribute ("rand", sRand);

钢铁男子

Maybe the above is wrong
should be
 String sRand = "";
Write this sentence in for